A ultrasonic diagnosis device provides a delay time distribution of received signals from a plurality of arrayed probe elements, forms ultrasonic beam having directivity to a predetermined direction and thereby constitutes a slice image of a body to be inspected. Since a human body is an inhomogeneous medium, it is necessary to vary the time delay distribution in accordance with a body to be inspected in order to form a high resolution ultrasonic beam.
The ultrasonic diagnosis device shown in FIG. 2 comprises probe elements 21 through 25 arranged at a predetemined position. The prove elements 21 through 25 transmit ultrasonic pulses to the body to be inspected and receive reflection pulses from the reflection member 61 disposed at opposition side of the body to be inspected. The reflection pulse signal is pulses that passed through the body to be inspected. For the sake of simplicity only signal receiving operation is explained. If a sound velocity in a medium is already known and uniform, a reflection pulse wave front from the reflection member 61 arrives to the probe elements 21 through 25 as an ideal wave front, and in this instance, due to the positional relationship between the reflection member 61 and the elements 21 through 25, the pulse wave front arrives earliest to the element 23 disposed near the reflection member 61 and arrives latest to the elements 21 and 25 far from the reflection member.
To improve the detection precision, it is necessary to manage the arrival time of all of the pulses to be at the same time, therefore, a proper delay is provided for the pulses received by the elements 22, 23 and 24. With this measure arrival time of all of the pulses is matched and thereafter by adding these pulses, only the received pulses from an aimed direction are amplified and, thereby a high resolution slice image is constituted. In this instance, if the body to be inspected is a homogeneous medium and the sound velocity therein is already known, a delay time to be provided is analytically obtained.
Now, when assuming that distances between the elements 21 through 25 are Li (1.ltoreq.i.ltoreq.5), a preset sound velocity of the ultrasonic diagnosis device is c, delay times to be provided for the elements 21 through 25 are .tau.i(1.ltoreq.i.ltoreq.5) and the maximum distance among Li (1.ltoreq.i.ltoreq.5) is Lmax, the delay time .tau.i is expressed by the following equation (1): EQU .tau.i=(Lmax-Li)/c (1)
Delay time 0 is provided for the elements 21 and 25 having the longest distance, the maximum delay time is provided for the element 23 having the shortest distance and proper delay times between the above two values are provided for the other elements 22 and 24 depending on the distances from the reflection member 61.
However, in practice there exists an inhomogeneous medium 64 between the elements 21 through 25 and the reflection member 61, therefore, the pulse wave front assumes a distorted wave front 63. Accordingly, the above delay time .tau.i is optimum as an initial delay time to be provided for the received signals of the respective elements, however, in order to obtain a high resolution slice image it is further necessary to provide a correction amount of delay time in view of the distorted wave front 63 for the above initial delay time. Further, in practice, a human body includes a variety of media such as muscle, fat and viscera, therefore, a further complex pulse wave front 63 is formed.
JP-A-1-135333 and IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 35, No. 6, November 1998, pp768-774 disclose a technical measure in which phase differences between signals of adjacent elements are determined by computing correlation between the signals of adjacent elements and the determined phase differences are used as the correction values for the above initial delay time.
In order to determine how far two signals having similar forms are separated, it is possible to determine the same by observing the correlation thereof. All of correlation factors are calculated by performing multiplications while shifting the signals, and based on a calculation of the shifting amount which shows the maximum correlation factor how the pulse waves are separated can be judged.
When determining the phase differences between the signals of adjacent elements by computing correlations therebetween, accurate phase differences are determined in a region where the correlation factors between signals are large, thereby a highly accurate correction can be effected. On the other hand, in a region where the correlation factors between the signals are small, the determined phase differences are inaccurate and the correction accuracy is reduced. Namely, it is necessary to use the phase differences which are computed in the region having a large correlation factor as the correction value for the initial delay time. However, the above patent and technical documents disclose no mechanism which feeds back the phase differences computed in a region having a high correlation as correction values for the initial delay time.
An object of the present invention is to resolve the above task and to provide a ultrasonic diagnosis device capable of obtaining a high resolution slice image and having a simple correlation factor computing circuit in which in order to remove influences of inhomogeneous medium in a living body a delay time distribution of received signals is modified by computing correlations between signals of adjacent elements as well as phase differences between the signals computed for a region having high correlations of the signals are automatically used as correction values for initial delay time distribution.